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Topic Review (Newest First)
08-02-2012 10:04 AM
DQ81
Quote:
Originally Posted by teledrew View Post
I have the same symptoms and have done same as you to try and solve. Did you ever find a solution, you left the thread without an end!
The carb needed a full rebuild.

hope that helps.

David
08-02-2012 03:27 AM
teledrew
Nothing worse...

Quote:
Originally Posted by DQ81 View Post
these were my initial thoughts.

fuel pump has been replaced in the past six months according to the previous owner. It does have that 'new pump' look, haha.

It does have an inline filter and I'm off to check the filter at the inlet.

I was thinking either filters or float level, feul starvation, because of it's behaviour.

Thanks for your help!

I'll let you know, nothing worse than a thread without an end.

I have the same symptoms and have done same as you to try and solve. Did you ever find a solution, you left the thread without an end!
03-14-2010 03:42 PM
Raufus You have gotten some good advice and I think you're on the right track. You should check/replace the filter first. Real common symptom. After that it almost sounds like your float plunger is sticking and not dropping, not letting the bowl fill with more fuel to keep up with demand, but that's just a WAG. Also, your rig is a 78 so your pickup in the gas tank could be clogged and sucking the strainer against the end of the pickup tube.
03-14-2010 12:31 PM
DQ81 these were my initial thoughts.

fuel pump has been replaced in the past six months according to the previous owner. It does have that 'new pump' look, haha.

It does have an inline filter and I'm off to check the filter at the inlet.

I was thinking either filters or float level, feul starvation, because of it's behaviour.

Thanks for your help!

I'll let you know, nothing worse than a thread without an end.
03-14-2010 12:19 PM
cobalt327
Quote:
Originally Posted by DQ81
Timing is fine, vac advance works fine.

8 BTDC, all in around 2.8k RPM @ 36 degrees.

This makes sense as like I said earlier:

If the truck idels a few minutes, punch it and accelerate moderately or WOT it's just fine for about 7 seconds and then stumbles almost completely stalls, let off the gas it recovers but if gas is re-applied it surges and won't accelerate or hold a speed properly.

The longer I stay off the gas the longer I can accelerate afterwards but it's back to surging pretty quickly. Timing isn't going to effect this.

It's a factory mechanical quadrajet with the correct timing.

What's next?
This is sounding like a fuel delivery problem.

See my reply above re the filter. There might also be an inline filter that has been added at some point- it too will need checked/replaced if needed.

The pump might be on its way out.
03-14-2010 12:17 PM
cobalt327
Quote:
Originally Posted by DQ81
how do i check the float on one of these carbs?

i'm very unfamiliar with them.

tia
Unless the top is removed, you can't actually measure the float level. You have to go by the way it behaves.

The floats can over time get gas logged and heavier than normal. This causes the level to be too high.

If the carb just began to misbehave, it might be a simple fix. If the carb is new to you or you don't know when it was last rebuilt, etc., it's a reasonable thing to expect to have to rebuild it.

You will also want to replace the fuel filter located at the fuel inlet/filter housing (1" size wrench to remove it). Hold the 1" from turning while you turn the fuel line fitting off w/a 5/8" fitting wrench (if it still has the OEM steel fuel line)- then remove the filter housing.

Be careful of the plastic gasket on the end of the housing- don't let it get between the threads when you reassemble it, and be careful not to overtighten or cross thread it.
03-14-2010 12:13 PM
DQ81 Timing is fine, vac advance works fine.

8 BTDC, all in around 2.8k RPM @ 36 degrees.

This makes sense as like I said earlier:

If the truck idels a few minutes, punch it and accelerate moderately or WOT it's just fine for about 7 seconds and then stumbles almost completely stalls, let off the gas it recovers but if gas is re-applied it surges and won't accelerate or hold a speed properly.

The longer I stay off the gas the longer I can accelerate afterwards but it's back to surging pretty quickly. Timing isn't going to effect this.

It's a factory mechanical quadrajet with the correct timing.

What's next?
03-14-2010 12:13 PM
nlamarche
New

To check the float your going to have to do a visual inspection. Considering the headaches your having, after confirming ignition is proper, Get a rebuild kit. for the carb. Find a large table, & an old white bed cover, Start your rebuild process. I have had this problem before. I cant answer what the actual problem was because sometimes its easier to just overhaul the system. As for accel. problems on inclines, these OEM carbs suck. You'll need to go to holley & by a truck kit. Otherwise a built 1406 or 1407 has always been my fav. Hope it helps.
03-14-2010 12:00 PM
DQ81 thanks, all good calls.

Heading out the door now with the timing light and going to check the vacuum canister etc.

how do i check the float on one of these carbs?

i'm very unfamiliar with them.

tia
03-14-2010 11:51 AM
cobalt327 Often carb problems, aren't carb problems.

What you might try, is see where the timing is at to start with. Initial, total and how the vac adv is hooked up- whether to ported or manifold vac.

Once that is straightened out, set the idle mixture screws w/a vac gage if possible, otherwise to the best idle speed w/both needles about (within 1/2 turn at the most) the same distance out.

Not until this is done, can any real attempt at tuning the carb can be done, although a lucky guess might pick out the problem from the possibilities.

Like float level, fuel pump, vac leaks, warped castings, incorrect power piston spring, jetting rods, secondary air valve setting etc.

If you just want to try something EZ- take a can of carb cleaner to it, following the can directions. It may loosen/clean the air bleeds and orifices enough to straighten it out.

Tighten (but don't OVER tighten the carb bolts. The 2 front bolts go all the way through the carb so be careful. Check that the screws securing the airhorn (and throttle body- but you need to pull the carb) for tightness.

Make sure the float is ABSOLUTELY not closing on you while the engine's running.

Let us know.
03-14-2010 11:32 AM
DQ81 thanks delawarebill just looking to get this carb running right.

I too like an Edelbrock and have a Holley vac. sec on my other truck so I hear ya.

Any thoughts on this one?
03-14-2010 11:21 AM
delawarebill
your carb size is ????

with your 350 u should be 600cfm or a little bigger won't hurt. i never did like a quad.. i'm using a edelbrock and love it.. if your doing alot of off roading u may want to looking holley's truck carb...
03-14-2010 11:01 AM
DQ81
Quadrajet on stock '78 350 idles fine drives like junk in truck

Stock 350 w/ factory Quadrajet in '78 4x4 Full Size truck.

Dual exhaust otherwise stock.

Start fine, takes a few more times turning over to start when warm.

Idles great.

If it has sat idling for 5 minutes or so. Take off WOT pull strong up to gear change into 2nd where it falls flat on it's face.

Give it a few seconds completely off the throttle I can accelerate again very shortly, falls on it's face again etc.

Can't keep a constant speed at any RPM and going up hill is a disastrous/maddening balance of feathering the throttle just to keep a MAX of 40mph, if I try WOT while driving it just about dies. Just very very very partial to no throttle keeps it alive.

Flat surface it's better but if I try and accelerate it does the same as mentioned above. The more load the worse the bog/cut out.

No backfires of any kind.

Fuel starvation?

Revs fine in Park, although haven't held it at higher RPM's for extended time to see if I can replicate the symptoms in park. Under load especially uphill its gets really really bad.

Look forward to knee-jerk reactions.

Thanks.

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